Choosing a good fabric is key for making a One Block Wonder. A print with a repeat of at least 12" is preferable. If the repeat is less, you'll need to buy 12 repeats instead of 6 (for hexagons), in order to have enough fabric to work with. The main print should be images of at least 2-6" in diameter. Many colors in the print are fine but not large areas of solid color. The background should not be a solid color. For example: if water is the background, then there should be swirls of waves or ripples with several colors, instead of a solid blue. Geometric prints or stripes generally don't work as well. A realistic print is the best, I think. Stylized flowers work also. I try to find a print with colors I like, knowing the final result will look totally different than the original fabric - so the print doesn't really matter. It could be chickens, hot peppers, fish, tropical birds, animals or an Oriental print with flowers,trees,people or cranes.
I also use a hinged mirror to preview fabrics at the store before I buy, to make sure it will work. Hold the mirror open on the width of the bolt, so you see 6 images in it & slowly slide it along the width of the fabric. This will give you an idea of possible hexagons you might get with that particular fabric.
Maxine Rosenthal has written 3 books on this topic - "One Block Wonders", "One Block Wonders Encore" and "One Block Wonders Cubed". The first two give good solid instructions for buying, preparing your fabric, cutting, piecing and sewing the strips together. The last one goes more into adding cubes, which is a whole other topic.